This project's overall objective is to define the relationship between the controlling effects of light and darkness on the turnover of photoreceptor membranes and those controls endogenously generated by neural or endocrine factors regulating adaptation and sensitivity. Because of their particularly high rate of membrane turnover compound eyes of higher crustaceans provide a particularly favorable preparation for studying the underlying mechanisms. Their similarities and differences relative to man and other vertebrates will aid in the basic understanding of vision. The goals of the current year's program are to continue developing the retinal cell dissociation techniques and the use of scanning electron microscopy to study receptor membrane cycling in the rhabdom. In so doing the particular aim is to provide new quantitative data on the pathways and mechanisms of breakdown and resynthesis of this precisely organized microvillus system. In addition transmission electron microscopy of crab and crayfish compound eyes will be continued in an effort to determine which components of membrane turnover are directly or indirectly dependent on external illumination and which are endogenously dependent on endocrine, nervous or nuclear regulation.